Recently pipelines have been acquiring increasing importance as long distance transportation means for crude oil, natural gas, etc. Up to now the American Petroleum Institute (API) Standards X80 and below have been applied to long-distance transportation main linepipes. However, higher-strength linepipes are required for (1) the improvement of transportation efficiency through increase of transportation pressure and (2) the improvement of laying efficiency through reduction of linepipe diameter and weight.
Particularly X120 grade linepipes having a tensile strength of 900 MPa or more and being capable of withstanding approximately twice as much internal pressure as X65 can transport approximately twice as much gas as same size linepipes of lower grades. Compared with methods which increase linepipes' pressure carrying capacity by increasing pipe wall thickness, the use of higher-strength linepipes realizes large savings in pipeline construction cost by saving costs of material, transportation and field welding work.
As has been already disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2000-199036, development of X120 linepipes, whose base material microstructure consists principally of a martensite/bainite mixture (lower bainite), is under way. However, the manufacture of this linepipe involves severe process constraints because extremely precise and strict microstructural control is required.
Increasing the strength of linepipes also necessitates increasing the strength of weld metal formed in joints between pipes field-welded (hereinafter referred to as field welds) in pipeline construction.
Generally the low-temperature toughness of the weld metal of welded joints is lower than that of the base metal and decreases further when the strength increases. Therefore, increasing the strength of linepipes necessitates increasing the strength of the weld metal of field welds, which leads to a lowering of low-temperature toughness.
If the strength of the weld metal of field welds is lower than the longitudinal strength of linepipe, strain concentrates in the field welds when stress occurs in the longitudinal direction of pipeline, thereby increasing the fracture susceptibility in heat-affected zone.
In ordinary pipelines, internal pressure generates circumferential stress but develops no longitudinal stress. However, in pipelines built in regions, such as discontinuous tundras, where the ground moves due to the actions of freezing and thawing, the movement of the ground bends pipelines and develops longitudinal stress.
That is, the weld metal of field welds of pipelines must have greater strength than the strength in the longitudinal direction of the pipe. However, the weld metal of field welds of the ultra-high-strength linepipes to which the present invention relates already has high strength. Therefore, further strengthening brings about a sharp decrease in toughness.
Accordingly, this problem will be relieved if the strength in the longitudinal direction of pipe that has no relation to the strength to withstand internal pressure is decreased while maintaining the strength in the circumferential direction of pipe.
The high-strength steel pipe the inventor proposed in Japanese Unexamined Patent Publication (Kokai) No. 2004-052104 differs in microstructure from the pipe according to this invention. This structural difference is due to differences in the amount of processing in the uncrystallized region and manufacturing conditions.